1. Field of the Invention
The present invention relates generally to the field of radio frequency (RF) power supplies. The present invention is useful in induction heating and plasma applications, but by no means is limited to such applications.
2. Related Art
Induction heating is a method of heating electrically conductive materials such as metals. Induction heating relies on, as the name implies, inducing electrical currents within a material to be heated. These induced currents, called eddy currents, dissipate energy and bring about heating. Common uses of induction heating include heat treating, welding, melting, packaging and curing. The number of consumer items which undergo induction heating during some stage of their production is large and rapidly expanding.
Prior to the development of induction heating, gas and oil-fired furnaces provided the prime means of heating metals and nonmetals. The advantages that induction heating offers over furnace techniques are numerous. For example, greater heating rates can be achieved by induction heating than can be achieved by gas or oil furnaces. Higher heating rates lead to shorter heating times, which lead to productivity increases and reduced labor costs. Furthermore, given today""s environmental concerns, induction heating is an attractive alternative to pollution producing furnaces.
The basic components of an induction heating system are (1) an AC power source (RF) power supply, (2) a tank circuit having an inductor coil and a capacitor, and (3) the material to be heated (a.k.a, xe2x80x9cworkpiecexe2x80x9d or xe2x80x9cloadxe2x80x9d). Common tank circuits used in induction heating are either parallel resonant or series resonant. A parallel resonant tank circuit includes a capacitance in parallel with the inductor coil and a series resonant tank circuit includes a capacitance in series with the inductor coil. A workpiece is heated by placing the workpiece within the inductor coil of the tank circuit and applying a high-power, RF alternating voltage to the tank circuit using the power supply. (The alternating voltage applied to the tank circuit causes an alternating current to flow through the inductor coil. The flow of an alternating current through the inductor coil generates an alternating magnetic field that cuts through the workpiece placed in the inductor coil. It is this alternating magnetic field that induces the eddy currents that heat the workpiece.)
A workpiece is heated most efficiently when the frequency of the alternating voltage applied to the tank circuit matches the tank circuit""s resonant frequency. That is, when the tank circuit (i.e., the tank circuit with a workpiece placed in the inductor coil) is driven at its resonant frequency, the transfer of power from the power supply to the workpiece is maximized. Thus, heating of the workpiece at the resonant frequency yields the greatest heating efficiency.
It should be noted that the resonant frequency of the tank circuit is in part determined by the characteristics of the inductor coil, such as the size and shape of the coil, and the characteristics of the workpiece when the workpiece is placed in the coil. Hence, moving the workpiece through the coil or altering the characteristics of the workpiece by heating it will change the resonant frequency of the tank circuit. Because the resonant frequency of the tank circuit changes as the workpiece is heated or moved through the coil, induction heating systems utilize a power supply having a tuning system for continuously tracking the resonant frequency of the tank circuit. By tracking the resonant frequency of the tank circuit, the power supply is better able to provide an alternating voltage that matches the resonant frequency, thereby efficiently heating the workpiece.
A problem with conventional induction power supplies, however, is that they operate over a limited frequency band. Another problem is that they are not capable of delivering a power into a load that is remotely located from the power supply. Therefore, what is desired is an RF power supply that overcomes the above and other limitations of conventional RF power supplies.
The present invention provides an RF power supply that is capable of quickly responding to varying load conditions so as to deliver the desired amount of power to the load. The RF power supply according to the present invention can track rapid changes in the resonant frequency of a tank circuit. The present invention also provides an RF power supply capable of delivering a wide range of power over a broad frequency range to a load that is remotely located from the power supply. The ability to deliver a wide range of power over a broad frequency range is a significant advantage because it enables an operator of the RF power supply to efficiently heat a wide variety of work pieces without having to change any components of the RF power supply.
According to one embodiment, the RF power supply includes a DC voltage source that provides a DC voltage within a predetermined voltage range; an amplifier, coupled to the DC voltage source, that provides an alternating voltage to a circuit connected to the RF output of the RF power supply; a frequency controller, coupled to the amplifier, to set the frequency of the alternating voltage produced by the amplifier; and a sensor, coupled to the circuit, to provide a signal to the frequency controller, where the frequency controller sets the frequency of the alternating voltage based on the signal received from the sensor. In one embodiment, the circuit is a tank circuit.
In one embodiment, the voltage source receives an AC voltage and converts it to a DC voltage. Preferably, the DC voltage source includes a pulse width modulator with hysteretic current mode control. The advantage of using such a pulse width modulator is that the DC voltage that is provided to the amplifier remains constant regardless of variations in the load and regardless of changing AC line voltages or frequencies. This is advantageous because the desired power level will be delivered to the load even when the load varies, regardless of changing AC line voltages or frequencies. Another significant advantage is that the DC voltage source according to a preferred embodiment is able to rapidly vary its output voltage over a wide range, thereby providing a means for rapidly varying the power delivered to the load over a wide power range.
In one embodiment, the frequency controller includes a processor and a frequency synthesizer. The processor receives a sensor signal from the sensor and, based on the received sensor signal, sends a frequency control signal to the frequency synthesizer. The frequency synthesizer outputs an alternating voltage, where the frequency of the alternating voltage is controlled by the frequency control signal. The output of the frequency synthesizer is coupled to the amplifier. The amplifier produces an alternating voltage having the same frequency as the frequency of the signal outputted by the frequency synthesizer. In this manner, the frequency controller sets the frequency of the alternating voltage produced by the amplifier. The advantage of using a processor and frequency synthesizer to set the frequency of the alternating voltage produced by the amplifier is that it enables the RF power supply to (1) quickly respond to varying load conditions; (2) operate over a wide range of frequencies; (3) easily adapt to series and parallel resonant tank configurations; and (4) easily adapt to a wide variety of resonant sensing schemes, such as an admittance, an impedance, a current, or a reflected power resonant sensing scheme.
In one embodiment, the sensor is an admittance sensor and the sensor signal provided to the frequency controller represents an admittance of the tank circuit. The admittance sensor provides numerous advantages. For example, (1) the admittance sensor enables the RF power supply to track the resonant frequency during rapid voltage ramp periods and over a broad frequency range, and (2) because the admittance sensor is tolerant of various waveshapes encountered in RF transmissions, the admittance sensor can be located at the tank circuit, thereby allowing remote sensing. In another embodiment, the sensor is a forward and/or reflected power sensor, and the signal provided to the frequency controller represents the forward power, the reflected power, the ratio of the forward to reflected power, or the ratio of the reflected to forward power.
The present invention additionally provides a unique method for delivering RF power to a load. The method quickly determines the resonant frequency of the tank circuit and is able to track rapid changes in the resonant frequency. These and further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.